The present invention is directed generally to a method and apparatus for detecting fires, and particularly to a method and apparatus for optically detecting fires using a combination of two or more wavelengths.
It is important that an optical fire detector is able to detect the presence of various types of flame in as reliable a manner as possible. This requires that the flame detector can discriminate between flames and other sources of infrared radiation. Commonly, optical flame detection is carried out in the infrared portion of the spectrum at around 4.5 .mu.m, a CO.sub.2 emission peak.
Simple flame detectors employ a single sensor, and a warning is provided whenever the signal sensed by the detector exceeds a particular threshold level. This simple approach suffers from false triggering, because it is unable to discriminate between flames and other bright objects, such as incandescent light bulbs, hot industrial processes such as welding, warm hands waved in front of the detector, and even sunlight.
Attempts have been made to overcome this problem by sensing radiation at two or more wavelengths. A comparison of the relative strengths of the signals sensed at each wavelength permits greater discrimination over false sources than when sensing at only a single wavelength.
Despite the implementation of detectors sensitive to radiation at more than one wavelength, optical detection techniques for detecting the presence of flames are still subject to high rates of false alarms, and misdiagnosis of true fires. For example, there is a difficulty in producing true alarms when monitoring fires at a long distance from the detector, say up to approximately 200 feet, when the signal to noise ratio is small. Also, fire detectors suffer from an inconsistency in fire detection characteristics under different fire conditions, for example fire temperature, size, position, fuel, and interfering background radiation. Consequently, there is a need for a fire detector whose ability to detect fires is less dependent on these factors.